Wireless communication systems are largely dependent on reliable transfer of packets. In case of transmission failure, the packet (or at least some information related to it) normally needs to be transmitted again. Of course, the most desirable scenario would be a communication system designed such that the need for retransmissions is eliminated. However, since it is impossible to always select the appropriate transmission power to ensure correct reception of a block, this is simply not feasible. For example, the maximum possible transmission power is typically limited and the power estimation often involves measurement errors, whereby the transmission error probability increases. There may also be unexpected interferers causing reception problems. Accordingly, retransmission mechanisms are essential for reliable data transmissions.
ARQ protocols are widely used in packet data communication systems to retransmit packets which have not been received correctly. There exist numerous ARQ protocols for wireless and wireline links, within the transport layer and also application layer protocols. Wireless link layer protocols comprising ARQ functionality are generally used in combination with Forward Error Correction (FEC) codes to ensure error-free delivery of received packets. This approach is commonly referred to as Hybrid ARQ (HARQ).
In accordance with ARQ and HARQ, the sending entity retransmits the data blocks after receiving a NACK feedback indicating transmission failures. Two different types of Hybrid ARQ can be distinguished. In HARQ Type I, the receiving entity discards the failed blocks immediately. Upon a NACK, the sender retransmits the data packet and the receiver tries again to decode the packet based on the retransmission. In HARQ Type II, the principle is instead to buffer the data blocks that were not received correctly and combine the buffered data with retransmissions. The soft combining procedure depends on which type of HARQ combining scheme that is used, e.g. Chase combining (HARQ-CC) or Incremental Redundancy (HARQ-IR).
Conventional ARQ protocols (including HARQ protocols) thus react with retransmissions when the sending entity receives a message from the peer ARQ entity that the data was not correctly received. To avoid protocol stalling there are normally also timers used at the sender to trigger a retransmission in case no feedback is received. Typically, such timers are started when the data unit is sent. If no feedback has been received within a predetermined period of time, a retransmission is initiated. In case feedback is received, the timer is stopped and depending on the feedback (positive or negative) it is decided whether there is to be a retransmission.
More specifically, there are three main ARQ protocol schemes, associated with different degrees of complexity. The simplest scheme is stop-and-wait ARQ. It allows sending one packet and waits for the feedback message, i.e., at the maximum one data packet is outstanding. Go-back-N ARQ allows to send up to N packets, but if a NACK is received it goes back to the negatively acknowledged packet and retransmits this and all subsequent packets regardless of whether they have been successfully received before or not. The most complex, but also best performing ARQ protocol is selective repeat ARQ. It uses a sliding window mechanism and can have also several packets outstanding at any point in time. Packets are positively or negatively acknowledged on an individual basis, e.g. by using bit maps in status messages. Typically selective repeat protocols require more complex timer solutions to protect against protocol stall conditions and to control unnecessary retransmissions.
The ARQ protocols operate with a certain round trip time, fixed or variable, which includes sending link layer data, processing at the receiver including generation of a response, transmitting the response, and processing the response at the data sender. Thus, every retransmission involves a delay associated with the round trip time. The fact that retransmissions increase the user perceived delay generally implies severe problems and leads to a degradation in data transfer performance.
Since retransmissions cannot be completely avoided, it would be very desirable to improve the quality and efficiency of the retransmissions and in particular to reduce retransmission delays. In this respect, the packet transfer of conventional telecommunication systems still is not entirely satisfactory and there is a need for an optimized retransmission mechanism.